There has been known a technique in which an adsorbent to adsorb water contained in an exhaust gas is arranged at a location upstream of an oxidation catalyst so that the temperature of the oxidation catalyst is caused to rise due to the generation of heat at the time when the water is adsorbed by the adsorbent (for example, see a first patent document).
Incidentally, there exists a limitation on the amount of water which can be adsorbed by the adsorbent. In addition, as the temperature of the adsorbent becomes higher, the amount of water which can be adsorbed by the adsorbent decreases. Accordingly, as the temperature of the adsorbent becomes higher, the water which has been adsorbed by the adsorbent will desorb. Thus, when the water is desorbed from the adsorbent, heat is deprived from the exhaust gas, and hence the temperature of the exhaust gas falls. As a result of this, there is a possibility that the temperature of the catalyst at the downstream side of the adsorbent may fall. When the temperature of the catalyst at the downstream side falls in this manner, purification performance for the exhaust gas decreases.
In addition, a plurality of catalysts may be arranged in an exhaust passage. Then, these catalysts may be arranged under the floor of a vehicle due to the restrictions on the installation thereof to the vehicle, etc. However, the distances from the adsorbent to the catalysts become long, so the temperature rise of catalysts at the downstream side becomes slow. In addition, as the heat is consumed by the temperature rise of the catalysts at the upstream side, the heat supplied to the catalysts at the downstream side will decrease, so the temperature rise of the catalysts at the downstream side becomes slow. When the temperature rise of the catalysts at the downstream side becomes slow in this manner, there will be a possibility that purification performance for the exhaust gas may be reduced. Moreover, depending on the kind of the catalysts, it is possible to enhance purification performance for the exhaust gas according to the manner of arrangement of the catalysts.
[First Patent Document] Japanese patent application laid-open No. 2002-138824
[Second Patent Document] Japanese patent application laid-open No. 8-93548
[Third Patent Document] Japanese patent application laid-open No. 10-159543
[Fourth Patent Document] Japanese translation of PCT international application No. 2006-512529
[Fifth Patent Document] Japanese patent application laid-open No. 5-31359
[Sixth Patent Document] Japanese patent application laid-open No. 2002-89240